The present invention relates to a voltage detecting circuit, and more particularly to a voltage detecting circuit suitable to be accommodated in various electronic circuit devices such as a single chip microcomputer, semiconductor memories such as read only memory and random access memory, and a digital signal processor.
In prior art, a low voltage detecting circuit is accommodated in a single chip microcomputer for detecting that a power voltage level is dropped into a lower voltage level than a predetermined reference voltage level. Almost all circuit configurations of the microcomputer are designed to be common in voltage level but only a power voltage is different in voltage level from the other circuit configurations. For example, the reference voltage is set at 4V and the power voltage level is set at 5V. The voltage detecting circuit may be designed to detect that the power voltage level is dropped to a lower voltage level than 4V of the reference voltage level. This voltage detecting circuit for detecting the lower voltage level than 4V is, however, inapplicable to a microcomputer operable in a lower power voltage of 3V because this voltage detecting circuit always detects the lower voltage level than 4V even the power voltage is normal at 3V. Once the voltage detecting circuit detects the lower voltage level than 4V, CPU CORE performs re-set process to prevent malfunction, whereby any other circuits in this microcomputer are also made inoperable by the re-set process.
In order to prevent this problem, the following two measures have been proposed.
The first measure is to set selectable two mask options. The first option is to place the low voltage detecting circuit into an operable state in accordance with the power voltage of the microcomputer. The second option is to set the low voltage detecting circuit into an inoperable state normally. If the above voltage detecting circuit for detecting the lower voltage level than 4V is applicable to the microcomputer operable with the power voltage of 5V, the voltage detecting circuit is set via the mask option to be operable state normally. If, however, the above voltage detecting circuit for detecting the lower voltage level than 4V is applicable to the microcomputer operable with the power voltage of 3V, the voltage detecting circuit is set via the mask option to be inoperable state normally.
The second measure is to set the low voltage detecting circuit to be inoperable state when the microcomputer is manufactured, so that in use of the microcomputer, CPU CORE sends an instruction to enable or unable operations of the lower voltage detecting circuit in accordance with the power voltage level. If the above voltage detecting circuit for detecting the lower voltage level than 4V is applicable to the microcomputer operable with the power voltage of 5V, CPU CORE sends the instruction to enable the low voltage detecting circuit to operate normally whereby the low voltage detecting circuit is placed into the operable state. If, however, the above voltage detecting circuit for detecting the lower voltage level than 4V is applicable to the microcomputer operable with the power voltage of 3V, CPU CORE sends the opposite instruction to unable the low voltage detecting circuit to not operate normally whereby the low voltage detecting circuit is placed into the inoperable state.
The above described first and second measures have the following problems respectively.
The problem with the first measure is inapplicable to other type microcomputer which is incapable of designating mask option. For example, a mask programmable ROM type microcomputer is capable of designating the above mask options. The above first measure is applicable to this microcomputer capable of designating the above mask options. RPROM type microcomputers allowing users to write programs in use into the RPROM are, however, incapable designating the above mask options. The above first measure is thus inapplicable to this microcomputer incapable of designating the above mask options.
The problem with the second measure is that it is difficult to make the programs common because CPU CORE sends the instruction to enable or unable the low voltage detecting circuit to operate in accordance with the power voltage level of the microcomputer whereby the program is required to be changed in accordance with the power voltage level.
In the above circumstances, it had been required to develop a novel voltage detecting circuit free from the above problem.